1. Field of the Invention
The present invention relates to a proton conductive inorganic material suitable for forming a fuel cell member such as an electrode for a fuel cell and an electrolyte membrane for a fuel cell, as well as to an electrolyte membrane, an electrode, a membrane electrode assembly and a fuel cell using the proton conductive inorganic material.
2. Description of the Related Art
Much research is presently being conducted on a proton conductive solid electrolyte for applying the proton conductive solid electrolyte to an electrochromic material or a sensor, and particularly, to a fuel cell having a high energy density that is operated under low temperatures.
A fuel cell comprises a proton conductive electrolyte membrane, a fuel electrode, which is also called an anode and mounted to one surface of the proton conductive electrolyte membrane, and an oxidizing agent electrode, which is also called a cathode and mounted to the other surface of the proton conductive electrolyte membrane. A fuel such as hydrogen or methanol is supplied to the fuel electrode. On the other hand, an oxidizing agent is supplied to the oxidizing agent electrode. The fuel is electrochemically oxidized in the anode so as to form protons and electrons. The protons thus generated in the anode are transmitted through the proton conductive electrolyte membrane so as to reach the cathode. As a result, water is formed in the cathode by the reactions carried out among the proton, the oxidizing agent and the electron supplied from an external circuit. Electricity is generated by the fuel cell by these reactions.
A membrane of an organic polymer material containing perfluorosulfonic acid, which is an organic polymer series ion exchange membrane, is known to the art as a proton conductive electrolyte membrane. To be more specific, NAFION (trade name) manufactured by Dupont Inc., which comprises tetrafluoroethylene-perfluoro vinyl ether copolymer as the base material and a sulfonic acid group as an ion exchange group, is known to the art as a proton conductive electrolyte membrane. Where an organic polymer material containing perfluoro sulfonic acid is used for forming an electrolyte membrane, the water contained in the membrane is decreased by the drying so as to lower the proton conductivity. As a result, a severe water supervision is required in the case where the electrolyte membrane is used in the vicinity of 100° C. at which a high output can be obtained so as to make the system highly complex. Also, the organic polymer material containing perfluoro sulfonic acid has a cluster structure so as to make the molecular structure sparse. As a result, the electrolyte membrane prepared by using the particular organic polymer material has a high permeability of an organic liquid fuel such as methanol so as to bring about a so-called “cross-over”, i.e., the phenomenon that the organic liquid fuel is migrated to reach the cathode. If the cross-over is generated, the organic liquid fuel reacts directly with the oxidizing agent so as to make it impossible to generate energy in the form of an electric power. The particular situation gives rise to the problem that it is impossible to obtain a stable output.
A metal oxide supporting sulfuric acid, which exhibits a solid super acidity, is known to the art as an inorganic solid acid-based ion exchange membrane, as disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 2002-216537. To be more specific, the metal oxide supporting sulfuric acid noted above is prepared by allowing the surface of an oxide containing at least one element selected from the group consisting of zirconium, titanium, iron, tin, silicon, aluminum, molybdenum and tungsten to support sulfuric acid, followed by applying a heat treatment to the oxide so as to fix the sulfuric acid to the surface of the oxide. The sulfate group fixed to the metal oxide exhibits the proton conductivity. However, in the case of the metal oxide, the sulfate group is liberated by the hydrolysis so as to lower the proton conductivity. It follows that the metal oxide supporting sulfuric acid is low in its stability when used as a material of the member of the fuel cell in which water is generated in the power generating process, particularly, when used as a material of the proton conductive electrolyte membrane included in a fuel cell using a liquid fuel. In other words, the metal oxide supporting sulfuric acid is not suitable for use for a stable power supply for a long time.
On the other hand, it is described in Japanese Patent Disclosure No. 2003-142124 that a metal oxide hydrate exhibiting a proton conductivity is used as a proton conductive substance. However, the structure of the metal oxide hydrate is shrunk, if the water of hydration is released by the drying caused by, for example, the power generation under high temperatures. What should be noted is that it is impossible to recover the original water of hydration even if water is supplied later so as to give rise to the problem that it is impossible to obtain a sufficient power generating performance.
PCT National Publication No. 2004-515351 (US 20040038105A) discloses an electrolyte membrane for a fuel cell, comprising an inorganic porous carrier and inorganic ionic conductor supported by the carrier. It is taught that the inorganic porous carrier is impregnated with an ionic liquid. To be more specific, it is taught that alumina particles are baked to a glass woven fabric used as the inorganic porous carrier by using a solution containing zirconia, followed by baking titania particles to the carrier by using a solution containing aluminum and vanadium.